Gravity Probe B Lifts Off

It happened a day later than planned, but Gravity Probe B is finally on its way to test an as-yet-unconfirmed prediction of Einstein's general theory of relativity. High winds delayed the launch yesterday from Vandenberg Air Force Base in California, but today the extraordinary $700 million satellite shot into orbit aboard a Delta II rocket.

When ships, satellites, and spacecraft need a stationary point to steer by, they take aim at the stars. Gravity Probe B is no exception. In this case, though, it would be more appropriate for the satellite to guide the guidestar. The craft is about to make a measurement so precise that the infinitesimal drift of its guidestar--a binary in the constellation Pegasus--would ruin the experiment. To orient the craft, astronomers on the ground will have to measure the star's motion and take it into account. Only then can Gravity Probe B perform its mission: to measure an as-yet-unseen consequence of Einstein's general theory of relativity known as "frame dragging."

According to relativity theory, the spinning Earth drags the fabric of space and time along with it. In the late 1950s, scientists realized that gyroscopes in an orbiting satellite would feel that twist as well. More than 4 decades later, Gravity Probe B is that satellite. Its heart is a concrete-mixer-sized Thermos bottle filled with liquid helium. Inside the Thermos are four incredibly smooth, spinning, golf-ball-sized quartz spheres--the most nearly perfect spherical objects ever created by humans. These spheres act as gyroscopes that, absent external influences, will always point in the same direction. "The gyros are one million times better than the best inertial navigation gyroscopes" used on vehicles such as rockets and airplanes, says Stanford physicist Francis Everitt, co-principal investigator of the Gravity Probe B mission.

This precision has made the satellite expensive and controversial, especially since few scientists expect any deviation from what Einstein's theory predicts. "If it's not agreeing with general relativity, there will be extreme excitement in the community," says California Institute of Technology physicist Kip Thorne. However, he says, even absent a surprise, the experiment is an important direct test of a prediction of relativity that has never been spotted before. With luck, Gravity Probe B should yield a result in a little more than 18 months. Says Anne Kinney, director of NASA's astronomy and physics division, "My expectation is that it will be in all the textbooks."